Fluorescent light emissions have previously been used to inspect electronic circuit boards consisting of layers of epoxy containing conductors defining circuits deposited therein. The conductors are generally metal that can have a very rough finish, such as copper, oxidized copper, reflowed copper with a tin covering and the like, wherein the rough surfaces prove to difficult properly optically image to inspect for defects. As a result, manufacturers of inspection equipment have employed lasers of wavelength suitable selectively fluoresce the epoxy layer carrying the conductors comprising the electronic circuit. In doing this, the epoxy background fluoresces in a different wavelength (color) while the conductors do not fluoresce and thus appear dark. If the conductors have defects such as broken conductors, such defects will accordingly appear as non-dark areas and can be located. Likewise, shorts appear dark where a bright region should be, and are thereby detected as defects. The importance for this use of the fluorescence is specifically to overcome the effects of the optical variations or imaging imperfections in the rough conductor lines themselves.
In an article entitled "Digital Optical Imaging Of Benzo-Cyclobritene (BCB) Thin Films On Silicon Wafers", by R. A. DeVries et al, Mat. Resi. Soc. Symp Proc. Vol 381, 1995, pages 165-173, thin films have been fluoresced to reveal thickness variations or particles and the like.
Fluorescence has also been used to detect various cracks and imperfections in machine parts and the like, wherein fluorescent material is washed on the part and then wiped off, so that the only place that the fluorescent material remains is in a crack or defect into which the fluorescent material has seeped.
In connection with the dense multi-layer integrated circuit parts of concern in connection with the present invention, on the other hand, where several layers or coatings are superimposed upon a bottom base substrate layer that itself may contain conductor lines and upon which a transparent or translucent intermediate insulation layer is superposed, in turn carrying a top layer containing patterns of conductors to be inspected for defects, a very different problem arises in the optical inspecting of such multi-layered parts for such defects and the like. This problem resides from the compounding of reflections from all the superposed layers since the reflected incident light reflects from the lower layers as well as the top layer, creating an overlay of all these reflections which does not allow discriminating inspection of just the top conductor pattern layer alone, as required.
Attempts to discriminate the top conductor pattern images by the use of dark fields, shallow angle illumination, color discrimination, and cross polarization illumination have not met with success.
Underlying the present invention, however, is the discovery that through the use of an intermediate light transmitting preferably translucent or transparent insulating layer that is susceptible, in response to a predetermined wavelength of incident light, to fluorescing at a different wavelength, the conductors of the conductor pattern of the top layer reflect the incident light at its original wavelength and do not fluoresce, and can be readily distinguished, and the desired inspection of the top layer conductors only can be discriminatingly and selectively achieved, while also rejecting, masking or making disappear, any incident light reflections from layers below the intermediate fluorescing layer, admirably solving the above-mentioned problem.